This invention relates to an air-fuel ratio feedback control device which controls the air-fuel ratio of the fuel mixture to be supplied to an engine by detecting the content of oxygen contained in the exhaust gas discharged from the engine.
The air-fuel mixture control device presently in use employs the following method to reduce the noxious components in the exhaust gas discharged from the engine. That is, the air-fuel ratio of the fuel mixture is controlled at a value near the theoretical air-fuel ratio by detecting the density of oxygen contained in the exhaust gas. This method enables an effective use of catalysts and is characterized by its ability to control the air-fuel ratio without being affected by changes in the external conditions such as atomospheric pressure and suction air temperature.
The control signals that are used in the conventional air-fuel ratio control device have two components: one is a sensitivity component whose magnitude is proportional to the level of the input signal based on a detected air-fuel ratio and the other is an integrated component obtained by integrating the input signal over time. Since the input signal takes either of two predetermined values--e.g., 4 volts when the mixture is rich and 0 volts when it is lean--and varies stepwise between these two values, the sensitivity component of the control signal proportional to the input signal has a constant voltage range. The time lag of the control system including the engine varies according to the quantity of the suction air. When the amount of air introduced into the engine is small, the time lag is large. When the amount of air introduced into the engine is large, the time lag is small. Thus, if the sensitivity component of the control signal proportional to the input signal is set for the operating range where the amount of suction air is small, the control may become too sensitive when a large amount of air is introduced, resulting in a low response speed. This may also cause a surging of the control system. On the contrary, if the sensitivity component of the control signal is set for the operating range where the quantity of air is large, the time lag may become excessive when only a small amount of air is introduced. This also decreases the response speed and the controllability of the system deteriorates.